Electric ignition system



Oct. 6, 1970 J. R. WILLSON 3,532,451

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- FUEL. VALVE FIG. 4 32 T M as HERMOCOUPLE \i 4k INVENTOR JAMES R. WILL$0N BY ATTORNEYS United States Patent O 3,532,451 ELECTRIC IGNITION SYSTEM James R. Willson, Garden Grove, Calif., assignor to Robertshaw Controls Company, Richmond, Va., a corporation of Delaware Filed Feb. 28, 1969, Ser. No. 803,233 Int. Cl. F23n /00 U.S. Cl. 431-66 12 Claims ABSTRACT OF THE DISCLOSURE An electric ignition system comprising an electric ignitor connected in series with an electromagnetic fuel valve and a negative temperature coeflicient thermistor which is mounted adjacent to the ignitor. When energized, the ignitor rapidly reaches ignition temperatures and causes the resistance of the thermistor to decrease thereby permitting increased current flow to the fuel valve to enable energization thereof. A second embodiment isolates the series circuit of the thermistor and the fuel valve from the ignitor by an isolation transformer, while a third embodiment provides such isolation by utilizing an electromagnetic relay and a separate power source. A still further embodiment replaces the thermistor and the separate power source of the third embodiment with a thermocouple.

BACKGROUND OF THE INVENTION The present invention relates to electrical ignition systems and, more particularly, to a rapid relight electric ignition system wherein the leakage of raw fuel produced by burner flame outage is eliminated.

In systems utilizing a pilot flame to ignite a fuel bur ner, it is conventional to include a safety device designed to rapidly effectuate fuel shut-off in the event of pilot flame outage. Such safety devices are extremely necessary since the pilot flame is subject to frequent outage due to drafts, dust and lint obstruction of the pilot fuel line, and the like. Most of the safety devices used in the prior art operate either by monitoring the electrical conductivity of the pilot flame or by sensing visible or invisible light emitted thereby. These devices are consequently quite complex and, in addition, require rapid acting fuel shut-off devices which increase production costs.

Electric ignitors are more desirable than pilot flame ignitors due to their reliability and immunity from drafts, etc.; however, they too have proved to be unsatisfactory in practical ignition systems unless accompanied by raw fuel leakage safety devices. The conventional safety devices recited above are both complex and expensive and require extensive modifications when used in conjunction with electric ignitors.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to construct a rapid relight electric ignition system having all the advantages of similarly employed prior art systems but utilizing fewer, and less expensive components than the components essential to such prior art systems.

Another object of this invention is to provide an electrict ignition system which produces rapid re-ignition in the event of burner flame outage.

This invention has a further object in the utilization of a negative temperature coefficient thermistor as a heat sensing element to enable the provision of a flow of fuel to a burner only when an electric ignitor is at fuel ignition temperatures.

An additional object of the present invention is the provision of an electric ignition system which utilizes a 3,532,451 Patented Oct. 6, 1970 ice shut-off time encountered during either fuel or electric power failure.

An advantage of this invention is the provision of a simple and reliable electric ignition system having inherent safety characteristics.

The present invention is summarized in that a rapid relight electric ignition system includes an electric ignitor, having a fuel ignition temperature heat-up tme, and an electro-mechanical fuel valve for establishing a flow of fuel upon electrical actuation thereof. The valve has a delay time before opening which is greater than the heat-up time of the electric ignitor, and has a rapid closure time. The system further includes a heat sensing device which is coupled to the fuel valve and located in close proximity to the electric ignitor for sensing the temperature of said electric ignitor and enabling actuation of the fuel valve when the electric ignitor is at fuel igniting temperatures, and a power source coupled to the ignitor, the heat sensing device, and the fuel valve for energization thereof.

These and other objects and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments of the invention when considered in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a schematic circuit diagram of the ignition system of the present invention;

FIG. 2 shows a schematic circuit diagram of a first modification of the present invention;

FIG. 3 shows a schematic circuit diagram of a second modification of the present invention; and

FIG. 4 shows a schematic circuit diagram of a third modification of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1, which illustrates a schematic circuit diagram of the ignition system of the present invention, shows an electric ignitor 10 coupled in series with a negative temperature coefiicient thermistor 12, a control switch 14, and an electromagnetic fuel valve 16. The series circuit is further coupled via switch 18 to a conventional power source represented by leads 20. The fuel valve 16 blocks the flow of fuel to the burner (not shown) when the system is inactive. When a current of sufficient value is supplied to the valve, however, it open to permit the fuel to pass therethrough. The electric ignitor 10 is designed to reach fuel ignition temperatures rapidly upon energization, while the fuel valve 16 is designed to have a delay time before opening which is greater than the heat-up time of the ignitor, and a rapid closure time. As will become more fully apparent as the disclosure proceeds, the ignitor 10, the fuel valve 16, and the thermistor 12 must be designed as a system in order to provide proper operation. Furthermore, as shown diagrammatically in the drawing, the system is constructed with thermistor 12 being located in close proximity to ignitor 10 so that its resistance will be proportional to and controlled by the temperature of the ignitor.

An additional resistor 22 is coupled in parallel with the series circuit formed by thermistor 12, switch 14, and valve 16 and provides a current shunt path around the fuel valve. Shunt resistor 22 is not necessary where ignitor 10 is, for example, a low mass device designed to produce fuel igniting temperatures when operated at low current levels, and is shown in the drawing merely for the purpose of completeness.

Referring now to FIG. 2, wherein like reference numerals are used to refer to similar components as used in FIG. 1, electric ignitor is coupled in series with the primary winding of an isolation transformer 24 across power source via switch 18. Transformer 24 may be a step-up transformer, a step-down transformer or a one-to-one transformer utilized solely for isolation. As in FIG. 1, a shunt resistor 22 is shown merely for the sake of completeness and is coupled across the primary winding of transformer 24. The secondary winding of transformer 24 is coupled to form a closed-loop series circuit with thermistor 12, switch 14, and fuel valve 16 to complete the illustrated embodiment.

In FIG. 3, the ignitor 10 is coupled in series with the energization winding of an electromagnetic relay 26 having a set of contacts '28 which are mechanically biased to an open or electrically discontinuous position (as illustrated) in the absence of energization of the relay winding. The thermistor 12 is coupled in series with contacts 28, fuel valve 16, and switch 14 across a second power source 30 which is electrically isolated from power source 20, as shown.

Referring now to FIG. 4, the ignitor 10, the energization winding of relay 26, and switch 18 are coupled in series across power source 20 as in the circuit of FIG. 3. However, in this embodiment contacts 28, fuel valve 16, and switch 14 are coupled to form a closed-loop series circuit with a thermocouple 32 which is designed to provide sufiicient current at fuel ignition temperatures to energize fuel valve 16. As in the above circuits employing a thermistor, the thermocouple is located in close proximity to the ignitor 10.

The circuits illustrated in the various figures all operate in a similar manner in accordance with the principles of operation of the system shown in FIG. 1. The system of FIG. 1 may be utilized for any of various fuel burner systems, however, its operation will be explained herein the context of a central heating furnace system. In such a heating system, switch 18 would correspond to the main power switch and could be operated either manually or automatically in accordance with the particular requirements of the installation. Closure of switch 18 connects the ignition system to the power source 20 thereby enabling energization of the system whenever there is a subsequent demand for heat. In the usual installation, switch 14, which could also be operated either manually or automatically, would respond to the operation of a thermostatically operated device (not shown) which, in turn, responds to the temperature of the area to be heated. The system of FIG. 1 is illustrated as being in an off condition permitting the ignitor 10 as well as thermistor 12 to remain at room temperature.

When the main switch 18 is turned to its closed or on position, current will flow through ignitor 10 and shunt resistor 22 causing the ignitor to rapidly heat up to fuel ignition temperatures. When a demand for heat exists, switch 14 will be closed thereby placing the series circuit of thermistor 12 and fuel valve 16 in parallel with resistor 22. Since the rapid heating of ignitor 10 causes the resistance of negative temperature coefiicient resistor 12 to decrease, the resistance of the fuel valve of the circuit will become smaller than the resistance of shunt resistor 22 thereby allowing an increasing amount of current to flow through the electromagnetic fuel valve 16. Ignitor 10, thermistor 12 and fuel valve 16 are designed so that the fuel valve current will become sufficiently large to cause its actuation only after the ignitor has reached fuel igniting temperatures. The valve 16 further provides a delay time between the time at which it is actuated and the time at which fuel flow therethrough commences, to thereby assure that the ignitor will be at fuel igniting temperatures prior to the establishment of fuel flow to the system burner (not shown).

4 After the damand for heat ceases, switch 14 will open the current path to the electromagnetic fuel valve 16 which then rapidly stops the flow of fuel to the burner. Since the ignitor current path through shunt resistor 22 still exists, the ignitor Will be maintained at its normal high temperature operating point so that reignition can occur, when the system recycles, as soon as fuel flow to the burner is reestablished.

If the ignitor fails and becomes an open circuit or if there is a failure of power source 20, the flow of current from the source 20 will cease and electromagnetic fuel valve 16 will rapidly close thereby preventing the leakage of raw fuel into the area surrounding the burner. When the power is subsequently reestablished, actuation of the fuel valve will be prevented by the high resistance of thermistor 12, which has been allowed to cool below fuel ignition temperatures, until the ignitor 10 has reached its normal high-temperature operating point. In the event that the ignitor becomes a short circuit, its temperature will rapidly decrease causing a like decrease in the temperature of thermistor 12 which produces an increase in the resistance of the series circuit formed by the thermistor and fuel valve 16. In this manner, even though the current path to the fuel valve is not directly broken, the increasing resistance in this path quickly reduces the current flow to the electromagnetic fuel valve 16 to the point where it can no longer remain actuated and therefore closes rapidly. Thus for conditions of open circuit or short circuit in the primary ignitor path, the leakage of raw fuel from the system is prevented. In addition since main switch 18 is always on when the system is operative, the ignitor always remains at fuel ignition temperatures. Therefore, any fuel interruption caused by fuel valve 16 or the fuel supply system (not shown) causing burner flame outage will cause a minimum interruption in service since the fuel will rapidly be reignited as soon as fuel fiow is reestablished.

The operation of the system illustrated in FIG. 2 is identical to that of the device of FIG. 1 except that the ignitor circuit is electrically isolated from the thermistor and the faul valve by transformer 24. In this modified circuit, the current utilized for actuation of the electromagnetic fuel valve 16 is induced in the secondary winding of transformer 24 when current if flowing through the primary winding thereof from power source 20. The induced current is regulated by thermistor 12 to provide raw fuel leakage prevention as well as rapid reignition in the manner described above with reference to FIG. 1.

The system illustrated in FIG. 3 also provides isolation and operates as follows. Upon closure of main switch 18, the ignitor 10 begins to heat up while the current flow through the energization winding of relay 26 causes the movement of contacts 28 to their closed position. A current path is now provided from power source 30 through thermistor 12, switch 14,'and fuel valve 16. This current path is only provided when the ignitor is on and will be regulated by thermistor 12 in the same manner as in the aforedescribed embodiments.

The system of FIG. 4 is substantially identical to that of FIG. 3 both in construction and operation. The system of FIG. 4, however, directly generates the temperature dependent current flow for actuation of fuel valve 16 by utilizing thermocouple 32 rather than the power source 30 and thermistor 12, shown in FIG. 3. In operation, the thermocouple 32 will produce sufiicient current to actuate fuel valve 16 only when it is at fuel ignition temperatures. At temperatures below that level the fuel valve will remain closed so that raw fuel leakage will be prevented.

The net result of the aforedescribed electric ignition systems constructed in accordance with the principles of operations of the present invention is the provision of inexpensive, yet reliable, electric ignition systems incorporating the features of rapid re-ignition following burner flame outage and raw fuel leakage prevention. This inven.

tion therefore makes practical the use of electrical ignition systems which were heretofore prohibitively complex and expensive.

Inasmuch as the present invention is subject to many variations, modifications and changes in detail, it is intended that all matter contained in the foregoing description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A rapid relight electric ignition system, comprising:

an electric ignitor having a fuel ignition temperature heat-up time;

an electromechanical valve for establishing a flow of fuel upon electrical actuation thereof, said valve having a delay time before opening greater than the heat-up time of said electric ignitor, and having a rapid closure time;

heat sensing means operatively connected with said fuel valve and said electric igniter and located in close proximity to said electric ignitor for sensing the temperature of said electric ignitor and enabling actuation of said fuel valve when said electric ignitor is a fuel igniting temperatures; and

a first power source coupled to said electric ignitor for energization thereof.

2. The invention as recited in claim 1 wherein said heat sensing means comprises a thermistor having a negative temperature coefiicient of resistance.

3. The invention as recited in claim 2 wherein said thermistor is electrically coupled in series with said electric ignitor and said fuel valve to form a series network, said series network being coupled across said first power source.

4. The invention as recited in claim 2, further including an isolation transformer having a primary winding coupled in series with said electric ignitor across said first power source, and a secondary winding coupled to form a closed-loop series circuit with said thermistor and said fuel valve.

5. The invenition as recited in claim 2, further including:

a relay, comprising:

an energization winding coupled in series with said electric ignitor across said first power source, and

a normally-open set of contacts adapted to be closed by said winding upon energization of said relay by said first power source, said contacts being coupled in series with said thermistor and said fuel valve to form a series network; and

a second power source coupled across said series network for electrical energization thereof.

6. The invention as recited in claim 1 wherein said heat sensing means comprises a thermocouple.

7. The invention as recited in claim 6, further including:

a relay having an energization winding coupled in series with said electric ignitor across said first power source, and a normally-open set of contacts adapted to be closed by said winding upon energization of said relay by said first power sources, said contacts being coupled to form a closed-loop series circuit with said thermocouple and said fuel valve.

8. In a rapid relight electric ignition system, the combination comprising:

an electric ignitor; and

control means coupled in series with said electric ignitor for establishing a flow of fuel after said ignitor has reached fuel ignition temperatures and for rapidly cutting off the flow of fuel when said electric ignitor is disabled, said control means comprising;

an electromechanical fuel valve, and

heat sensing means located adjacent said ignitor and electrically coupled in series with said fuel valve for enabling actuation of said fuel valve when said electric ignitor is at fuel igniting temperatures; and

a power source coupled to said ignitor and said control means for energizing the same.

9. The invention as recited in claim 1 further including:

a relay having an energization winding coupled in series with said electric ignitor, and a set of normally-open contacts adapted to be closed upon energization of said winding, said contacts being coupled in series with said heat sensing means and said fuel valve.

10. The invention as recited in claim 9, wherein said heat sensing means comprises a thermocouple for generating a sufficient amount of electrical power when at fuel igniting temperatures to actuate said electromechanical fuel valve.

11. The invention as recited in claim 9, wherein said heat sensing means comprises a thermistor having a negative temperature coefficient of resistance.

12. The invention as recited in claim 8, wherein said heat sensing means comprises a thermistor having a high value of electrical resistance at temperatures below fuel ignition temperatures thereby inhibiting actuation of said fuel valve and having a low value of electrical resistance at fuel ignition temperatures thereby enabling actuation of said fuel valve.

References Cited UNITED STATES PATENTS 2,747,799 5/1956 Schultz et al. 3,151,661 10/1964 Matthews 43l66 X FOREIGN PATENTS 650,237 10/1962 Canada.

CARROLL B. DORITY, 111., Primary Examiner 

